It is generally known that maintaining laminar flow of air passing over an airfoil can improve the aerodynamics and performance of an aircraft. For example, it is known that delaying the transition of boundary layer airflow from laminar flow to turbulent flow over aerodynamic surfaces can reduce skin friction and reduce aerodynamic drag. One method of delaying the transition of airflow from laminar to turbulent flow is by installing a porous skin at critical areas of an airfoil such as along the leading edges of wings, tail surfaces and engine nacelles. The porous skin typically includes a large quantity of apertures or pores of relatively small size. The porous skin may also include narrow slots or elongated pores to provide porosity. In one example, the pores in the porous skin of a wing leading edge may be formed at diameters on the order of several thousandths of an inch (e.g., 0.0025″) or less and at spacings of tens of thousandths of an inch (e.g., 0.035″) between adjacent pores.
By applying a suction force to the porous skin, boundary layer airflow that is attached to the airfoil (i.e., along the attachment line) may be drawn through the pores to stabilize the boundary layer against small disturbances which may grow and ultimately lead to early transition turbulence. The application of the suction force thins the boundary layer velocity profiles. The net result is a delay in boundary-layer transition, a decrease in skin friction drag, and an increase in aerodynamic efficiency of the aircraft. The increase in aerodynamic efficiency may be especially noticeable at cruise altitudes for long distance flights wherein significant fuel savings may be achievable as a result of reduced aerodynamic drag.
One of the challenges preventing widespread implementation of laminar flow control systems of the suctioning type is the requirement of a relatively large suction force. The suction force must be sufficiently large to draw the boundary layer air through the porous skin. In addition, the suction force must be large enough to duct the suctioned air to another location on the aircraft for discharge into the external atmosphere. Prior art attempts at developing a suctioning system with a sufficiently large suction force have resulted in active suctioning systems that rely on pumps such as compressors to generate the suction force. Active suctioning systems may also rely on engine bleed air drawn from engine compressors or other turbo-machinery to provide the suction force. While generally effective for their intended purposes, active suctioning systems typically require a variety of flow ducts, control valves and other components that add to the weight, complexity and cost of the aircraft and detract from the aircraft operating efficiency.
Another challenge preventing widespread implementation of laminar flow control systems having porous skins is contamination or blockage of pores which can occur under certain conditions. Such contamination may include atmospheric contamination and/or manmade contamination which may reduce the effectiveness of laminar flow control systems. For example, during takeoff and climb-out of an aircraft fitted with porous skins, precipitation in the form of rain or moisture in low-altitude clouds can fill the pores with water that will later freeze as the aircraft climbs into colder air. The frozen moisture blocks the pores and reduces the effectiveness of the suctioning system in maintaining laminar flow over the aircraft during cruise. Manmade contamination such as de-icing fluids applied during ground operations may also reduce the effectiveness of the laminar flow control system by clogging the pores with de-icing fluid.
The accumulation of frost on an aircraft may also reduce the effectiveness of a suctioning system by blocking the pores. Although frost accumulations on exterior surfaces of the porous skin may eventually sublimate away, moisture or liquid on the interior surfaces of the porous skin may become trapped in the pores and will remain due to the relatively small amount of surface area over which the sublimation would otherwise occur. Furthermore, local flow velocities inside the pores are relatively low and therefore insufficient to overcome surface tension resistance of the moisture trapped within the pores.
Prior art attempts at preventing clogging of pores include active purging systems wherein pressurized air is expelled or discharged outwardly through the pores. Such purging systems may be activated prior to takeoff in anticipation of rain or moisture-laden clouds that an aircraft may encounter during climbout. By discharging air through the pores, purging systems maintain the pores in an unblocked state and prevent the freezing of residual liquid that may be trapped within the pores. Although effective for their intended purposes, prior art purging systems suffer from several defects that detract from their overall utility.
For example, all known purging systems for use with suctioning-type laminar flow control systems are of the active type. Active purging systems require energy input into the air on the interior side of the porous skin in order to pressurize the air such that the air may be discharged out of the pores. As in the case with active suctioning systems, pressurized air for active purging systems may be provided by engine compressors or other pumping machinery or may be drawn from engine bleed air. For example, pressurized air for an active purging system may be provided by tapping a portion of the bypass flow of a high-bypass turbofan engine.
As may be appreciated, the system architecture of an active purging system such as one which draws pressurized air from an aircraft engine may be functionally and structurally complex. In addition, the installation of components and machinery for providing the pressurized air adds to the complexity and cost of the aircraft. Furthermore, additional components of an active purging system may increase the weight of the aircraft which may detract from gains in fuel efficiency otherwise attainable with the laminar flow control system.
Even further, aircraft such as commercial airliners are increasingly fabricated without significant bleed air extraction from the engine. Although bleed air has been conventionally used for cabin pressurization and in-flight de-icing, modern aircraft are increasingly employing electrical power as a substitute for conventional engine-generated pneumatic power (i.e., bleed air) in order to maximize the amount of pneumatic power that is available to the engines for producing thrust. As such, conventional engine bleed air may be unavailable on future aircraft for providing pressurized air for use with active purging or suctioning.
As can be seen, there exists a need in the art for a simple, low-cost means for eliminating the need for pumping machinery conventionally associated with active purging and suctioning of a laminar flow control system.